NOx Suppression Techniques for an Indurating Furnace

ABSTRACT

Techniques for suppressing NOx in an indurating furnace include the use of a premix burner, the use of a staged fuel injector structure to enhance fuel lean conditions in the downdraft, and the use of a Venturi mixture structure in the downcomer.

RELATED APPLICATIONS

This application claims the benefit of provisional U.S. patent application 61/180,235, filed May 21, 2009, and provisional U.S. patent application 61/162,853, filed Mar. 24, 2009, both of which are incorporated by reference.

TECHNICAL FIELD

This technology relates to a heating system in which combustion produces oxides of nitrogen (NOx), and specifically relates to a method and apparatus for suppressing the production of NOx in an indurating furnace.

BACKGROUND

Certain industrial processes, such as heating a load in a furnace, rely on heat produced by the combustion of fuel and oxidant. The fuel is typically natural gas. The oxidant is typically air, vitiated air, oxygen, or air enriched with oxygen. Combustion of the fuel and oxidant causes NOx to result from the combination of oxygen and nitrogen.

An indurating furnace is a particular type of furnace that is known to produce high levels of NOx. Large quantities of pelletized material, such as pellets of iron ore, are advanced through an indurating process in which they are dried, heated to an elevated temperature, and then cooled. The elevated temperature induces an oxidizing reaction that hardens the material. When cooled, the indurated pellets are better able to withstand subsequent handling in storage and transportation.

The indurating furnace has sequential stations for the drying, heating, and cooling steps. Pelletized material is conveyed into the furnace, through the sequential stations, and outward from the furnace. Air shafts known as downcomers deliver downdrafts of preheated air to the heating stations. Burners inject fuel and combustion air into the downdrafts, and the resulting combustion provides heat for the reaction that hardens the pelletized material.

An example of a pelletizing plant 10 with an indurating furnace 20 is shown schematically in FIG. 1. A movable grate 24 conveys loads of pelletized material 26 into the furnace 20, through various processing stations within the furnace 20, and then outward from the furnace 20. The processing stations include drying, heating, and cooling stations. In this particular example, the drying stations include an updraft drying station 30 and a downdraft drying station 32. The heating stations include preheat stations 34 and firing stations 36. First and second cooling stations 38 and 40 are located between the firing stations 36 and the furnace exit 42. Burners 44 are arranged at the preheating and firing stations 34 and 36.

A blower system 50 drives air to circulate through the furnace 20 along the flow paths indicated by the arrows shown in FIG. 1. As the pelletized material 26 advances from the firing stations 36 toward the exit 42, it is cooled by the incoming air at the first and second cooling stations 38 and 40. This causes the incoming air to become heated before it reaches the burners 44. The preheated air at the second cooling station 40 is directed through a duct system 52 to the updraft drying station 30 to begin drying the material 26 entering the furnace 20. The preheated air at the first cooling station 38, which is hotter, is directed to the firing and preheat stations 36 and 34 through a header 54 and downcomers 56 that descend from the header 52. Some of that preheated air, along with products of combustion from the firing stations 36, is circulated through the downdraft drying station 32 before passing through a gas cleaning station 58 and onward to an exhaust stack 60.

As shown for example in FIG. 2, each downcomer 54 defines a vertical passage 61 for directing a downdraft 63 from the header 52 to an adjacent heating station 36. Each burner 44 is arranged to project a flame 65 into a downcomer 54. Specifically, each burner 44 is mounted on a downcomer wall 66 in a position to project the flame 65 in a direction extending across the vertical passage 61 toward the heating station 36 to provide heat for the reaction that hardens the pelletized material 26.

The burner 44 of FIG. 2 is an inspirating burner, which injects fuel and combustion air. The combustion air includes unheated air from the blower assembly 50 and preheated air that is drawn from the downdraft 63 through an inspirator 68. The fuel and combustion air are typically injected at a fuel-rich ratio. This produces high levels of interaction NOx as the unmixed or poorly mixed fuel interacts with the high temperature downdraft air.

SUMMARY OF THE INVENTION

The invention provides techniques for suppressing the production of NOx in an indurating furnace. The furnace has a downcomer with a vertical passage that directs a downdraft toward a heating station. A burner injects fuel gas and combustion air into the downcomer for combustion to occur in the downdraft. The NOx suppression techniques include the use of a premix burner, which injects a premix of fuel gas and combustion air into the downcomer. This avoids the production of NOx that would occur upon interaction of unmixed fuel gas with preheated downdraft air. The NOx suppression techniques further include the use of a staged fuel injector structure to enhance fuel-lean conditions in the downdraft, and the use of a Venturi mixture structure in the downcomer. Any one or more of these techniques may be used to suppress the production of NOx.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a pelletizing plant including an indurating furnace known in the prior art.

FIG. 2 is an enlarged partial view of parts of the indurating furnace of FIG. 1.

FIG. 3 is a schematic view similar to FIG. 2, but shows parts of an indurating furnace configured according to the invention.

Each of FIGS. 4 through 9 is a schematic view similar to FIG. 3, but shows an alternative embodiment of the invention.

DETAILED DESCRIPTION

The structures shown schematically in FIGS. 3-9 are examples of how a person of ordinary skill in the art can make and use the claimed invention. These examples are described here to meet the enablement and best mode requirements of the patent statute without imposing limitations that are not recited in the claims. The various parts of the illustrated structures, as shown, described and claimed, may be of either original and/or retrofitted construction as required to accomplish any particular implementation of the invention.

As shown partially in FIG. 3, an indurating furnace 100 is equipped with burners 102 that are configured according to the invention. The furnace 100 also has a reactant supply and control system 104 for operating the burners 102 according to the invention. The furnace 100 is otherwise the same as the furnace 20 described above, with downcomers 110 defining vertical passages 111 for directing downdrafts 113 from a header to adjacent heating stations 114. Each burner 102 is mounted on a corresponding downcomer wall 116 in a position to project a premix flame 119 into the downdraft 113 in a direction extending toward the heating station 114. This provides heat for a reaction that hardens pelletized material 124 on a movable grate 126 at the heating station 114.

The burners 102 are preferably configured as premix burners with the structure shown in FIG. 3. This burner structure has a rear portion 140 defining an oxidant plenum 141 and a fuel plenum 143. The oxidant plenum 141 receives a stream of unheated atmospheric air from a blower system 144. The fuel plenum 143 receives a stream of fuel from the plant supply of natural gas 146.

Mixer tubes 148 are located within the oxidant plenum 141. The mixer tubes 148 are preferably arranged in a circular array centered on a longitudinal axis 149. Each mixer tube 148 has an open inner end that receives a stream of combustion air directly from within the oxidant plenum 141. Each mixer tube 148 also receives streams of fuel from fuel conduits 150 that extend from the fuel plenum 143 into the mixer tube 148. These streams of fuel and combustion air flow through the mixer tubes 148 to form a combustible mixture known as premix.

An outer portion 160 of the burner 102 defines a reaction zone 161 with an outlet port 163. The premix is ignited in the reaction zone 161 upon emerging from the open outer ends of the mixer tubes 148. Ignition is initially accomplished by use of an igniter before the reaction zone 161 reaches the auto-ignition temperature of the premix. Combustion proceeds as the premix is injected from the outlet port 163 into the downcomer 110 to mix with the downdraft 113. The fuel in the premix is then burned in a combustible mixture with both premix air and downdraft air. By mixing the fuel with combustion air to form premix, the burner 102 avoids the production of interaction NOx that would occur if the fuel were unmixed or only partially mixed with combustion air before mixing into the downdraft air.

As further shown in FIG. 3, the reactant supply and control system 104 includes a duct 180 through which the blower system 144 receives unheated air from the ambient atmosphere. Another duct 182 extends from the blower system 144 to the oxidant plenum 141 at the burner 102. A fuel line 184 communicates the fuel source 146 with the fuel plenum 143 at the burner 102. Other parts of the system 104 include a controller 186, oxidant control valves 188, and fuel control valves 190.

The controller 186 has hardware and/or software that is configured for operation of the burner 102, and may comprise any suitable programmable logic controller or other controlled device, or combination of controlled devices, that is programmed or otherwise configured to perform as recited in the claims. As the controller 186 carries out those instructions, it operates the valves 188 and 190 to initiate, regulate, and terminate flows of reactant streams that cause the burner 102 to fire the premix flame 119 into the downcomer 110. The controller 186 is preferably configured to operate the valves 188 and 190 such that the fuel and combustion air are delivered to the burner 102 in amounts that form premix having a lean fuel-to-oxidant ratio. The fuel-lean composition of the premix helps to avoid the production of interaction NOx in the downdraft 113.

Although the premix produces less interaction NOx upon combustion of the fuel-air mixture in the high temperature downdraft 113, this has an efficiency penalty because it requires more fuel to heat the cold atmospheric air in the premix. The efficiency penalty is greater if the premix has excess air to establish a lean fuel-to-oxidant ratio. However, the efficiency penalty can be reduced or avoided by using an embodiment of the invention that includes preheated air in the premix. For example, in the embodiment shown in FIG. 4, the reactant supply and control system 104 includes a duct 200 for supplying the burner 102 with preheated downdraft air from the downcomer 110. As in the embodiment of FIG. 3, the controller 186 in the embodiment of FIG. 4 is preferably configured to operate the valves 188 and 190 such that the fuel gas, the unheated air, and the preheated air are delivered to the burner 102 in amounts that form premix having a lean fuel-to-oxidant ratio.

The embodiment of FIG. 5 also reduces the efficiency penalty caused by the premix in the embodiment of FIG. 3. In this embodiment, the reactant supply and control system 104 includes a fuel branch line 206 with a control valve 208. As shown schematically, the branch line 206 terminates at a fuel injection port 210 that is spaced axially downstream from the burner 102. The reactant supply and control system 104 is thus configured to supply primary fuel gas and combustion air to the premix burner 102, and to separately inject second stage fuel gas into the downcomer 110 without combustion air. The controller 186 is preferably configured to operate the valves 188, 190 and 208 such that primary fuel and combustion air are delivered to the burner 102 in amounts that form premix having a lean fuel-to-oxidant ratio, while simultaneously providing the branch line 206 with second stage fuel in an amount that is stoichiometric with the premix supplied to the burner 102. Since the premix in this embodiment includes less than the total amount of fuel, it can include a correspondingly lesser amount of unheated air to establish a lean fuel-to-oxidant ratio. The lesser amount of unheated air in the premix causes a lower efficiency penalty.

An additional NOx suppression feature of the invention appears in FIG. 5 where the downcomer 110 is shown to have a recessed wall portion 220. This portion 220 of the downcomer 110 defines a combustion zone 221 that is recessed from the vertical passage 111. The burner 102 is mounted on the recessed wall portion 220 of the downcomer 110 so as to inject premix directly into the combustion zone 221 rather than directly into the vertical passage 111.

In the embodiment of FIG. 5, the premix flame 119 projects fully through the combustion zone 221 and into the vertical passage 111. The controller 186 could provide the burner 102 with fuel and combustion air at lower flow rates to cause the premix flame 119 to project only partially through the combustion zone 221 and thereby to produce less interaction NOx in the vertical passage 111. As shown in FIG. 6, a deeper combustion zone 225 could have the same effect without reducing the reactant flow rates.

Additional suppression of interaction NOx can be achieved with differently staged fuel injection ports along with a recessed combustion zone. As shown for example in FIG. 7, these may include a port 230 for injecting staged fuel directly into the recessed combustion zone 225, a port 232 for injecting staged fuel directly into the vertical passage 111 upstream of the recessed combustion zone 225, and a port 234 for injecting staged fuel into the vertical passage 111 at a location downstream of the recessed combustion zone 225.

The embodiment of FIG. 8 has another alternative arrangement of staged fuel injector ports 236. These ports 236 are all arranged on the downcomer wall 116 in positions spaced radially from the burner port 163, and are preferably arranged in a circular array centered on the burner axis 149. The reactant supply and control system 104 includes a staged fuel control valve 238 for diverting fuel to a manifold 240 that distributes the diverted fuel to each port 236 equally. The ports 236 together inject that fuel into the downcomer 110 in a circular array of second stage streams. The ports 236 may be configured to inject the second stage fuel streams in directions that are parallel to and/or inclined toward the axis 149.

In the embodiment of FIG. 9, the downcomer 110 is equipped with a Venturi mixer structure 250. The Venturi mixture structure 250 has a mixer flow passage 251, and is arranged within the vertical downcomer passage 111 such that the mixer flow passage 251 is aligned with the burner port 163. The reactant supply and control system 104 has a staged fuel injector port 252 for injecting second stage fuel without combustion air at a location upstream of the Venturi mixture structure 250. It also has a staged fuel injector port 254 for injecting third stage fuel without combustion air at a location downstream of the Venturi mixer structure 250. In this arrangement, the premix injected from the burner port 163 entrains both downdraft air and second stage fuel into the mixer flow passage 251. This promotes thorough mixing of those reactants for uniform combustion, and helps to suppress the peak flame temperature to suppress the production of NOx. Fuel efficiency can be improved by providing the staged fuel in an amount that is stoichiometric with the premix.

This written description sets forth the best mode of carrying out the invention, and describes the invention so as to enable a person skilled in the art to make and use the invention, by presenting examples of elements recited in the claims. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they have equivalent elements with insubstantial differences from the literal language of the claims. 

1. An apparatus comprising: an indurating furnace structure defining drying, heating, and cooling stations; a conveyor to convey pelletized material through the drying, heating and cooling stations; a downcomer configured to direct a downdraft toward the heating station; and a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft.
 2. An apparatus as defined in claim 1 further comprising a reactant supply and control system configured to supply the premix burner with fuel gas and combustion air in amounts for premix to have a lean fuel-to-oxidant ratio.
 3. An apparatus as defined in claim 1 further comprising a reactant supply and control system configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air.
 4. An apparatus as defined in claim 3 wherein the reactant supply and control system is configured to supply the premix burner with the fuel gas, the unheated air, and the heated downdraft air in amounts for the premix to have a lean fuel-to-oxidant ratio.
 5. An apparatus as defined in claim 1 wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the burner is configured to inject the premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage.
 6. An apparatus as defined in claim 5 further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage.
 7. An apparatus as defined in claim 5 further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone.
 8. An apparatus comprising: an indurating furnace structure defining drying, heating, and cooling stations; a conveyor to convey pelletized material through the drying, heating and cooling stations; a downcomer configured to direct a downdraft toward the heating station; a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft; and a reactant supply and control system configured to supply fuel gas and combustion air to the premix burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the premix injected by the premix burner.
 9. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the premix burner.
 10. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the premix burner.
 11. An apparatus as defined in claim 8 wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the premix burner is configured to inject premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage.
 12. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage.
 13. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone.
 14. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the recessed combustion zone.
 15. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the recessed combustion zone.
 16. An apparatus as defined in claim 8 further comprising a Venturi mixer structure that has a mixer flow passage and is arranged for the injected premix to entrain staged fuel gas into the mixer flow passage.
 17. An apparatus as defined in claim 16 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure.
 18. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to supply fuel gas and combustion air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air at a rate that is stoichiometric with the premix supplied to the premix burner.
 19. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air.
 20. An apparatus as defined in claim 19 wherein the reactant supply and control system is configured to supply the fuel gas, the unheated air, and the heated downdraft air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air into the downcomer at a rate that is stoichiometric with the premix supplied to the premix burner.
 21. An apparatus as defined in claim 8 wherein the premix burner port has a central axis and is configured to inject the premix into the downdraft along the axis, and the reactant supply and control system comprises an array of secondary fuel ports that are spaced radially from the burner port and configured to inject streams of second stage fuel gas into the downdraft in directions parallel to or inclined toward the axis.
 22. An apparatus comprising: an indurating furnace structure defining drying, heating, and cooling stations; a conveyor to convey pelletized material through the drying, heating and cooling stations; a downcomer configured to direct a downdraft toward the heating station; a burner configured to inject fuel gas and combustion air into the downcomer for combustion in the downdraft; a reactant supply and control system configured to supply fuel gas and combustion air to the burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the burner; and a Venturi mixer structure that has a mixer flow passage and is arranged for the injected fuel gas and combustion air to entrain staged fuel gas into the mixer flow passage.
 23. An apparatus as defined in claim 22 wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure.
 24. An apparatus as defined in claim 22 wherein the burner is a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer, and the reactant supply and control system is configured to supply the burner with fuel gas and combustion air in amounts for the premix to have a lean fuel-to-oxidant ratio while simultaneously injecting the staged fuel gas at a rate that is stoichiometric with the premix supplied to the burner. 